Is "litotes" a singular or plural noun? How is the word used in a sentence?
It is a singular noun.
According to Oxford Dictionary of English 'litotes' is a singular noun.
A definition can be an example of use in the sentence:
Litote is an ironical understatement in which an affirmative is expressed by the negative of its contrary (e.g. I shan't be sorry for I shall be glad).